Oil sand, as known in the Fort McMurray region of Alberta, comprises water-wetted sand grains having viscous bitumen flecks trapped between the grains. It lends itself to separating or dispersing the bitumen from the sand grains by slurrying the as-mined oil sand in water so that the bitumen flecks move into the aqueous phase.
The bitumen in McMurray oil sand has been commercially recovered for the past 25 years using the following general scheme (referred to as the "hot water process"):
dry mining the oil sand at a mine site that can be kilometers from an extraction plant; PA1 conveying the as-mined oil sand on conveyor belts to the extraction plant; PA1 feeding the oil sand into a rotating tumbler where it is mixed for a prescribed retention time with hot water (80.degree. C.), steam, caustic and naturally entrained air to yield a slurry typically having a temperature of 80.degree. C. The bitumen flecks are heated and become less viscous. Chunks of oil sand are ablated or disintegrated. The sand grains and bitumen flecks are dispersed or separate in the water. To some extent bitumen flecks coalesce and grow in size. They may contact air bubbles and coat them to become aerated bitumen. The term used to describe this overall process in the tumbler is "conditioning"; PA1 the slurry produced is then diluted with additional hot water and introduced into a large, open-topped, conical-bottomed, cylindrical vessel (termed a primary separation vessel or "PSV"). The diluted slurry is retained in the PSV under quiescent conditions for a prescribed retention period. During this period, the aerated bitumen rises and forms a froth layer which overflows the top lip of the vessel and is conveyed away in a launder; and the sand grains sink and are concentrated in the conical bottom--they leave the bottom of the vessel as a wet tailings stream. Middlings, a watery mixture containing solids and bitumen, extend between the froth and sand layers. The tailings and middlings are withdrawn, combined and sent to a secondary flotation process carried out in a deep cone vessel wherein air is sparged into the vessel to assist with flotation. This vessel is referred to as the TOR vessel. It and the process conducted in it are disclosed in U.S. Pat. No. 4,545,892, incorporated herein by reference. The bitumen recovered is recycled to the PSV. PA1 supplying heated water at the mine site; PA1 mixing the dry as-mined oil sand with the heated water at the mine site in predetermined proportions using a device known as a "cyclofeeder", to form a slurry of controlled density having a temperature in the order of 50.degree. C.; PA1 screening the slurry to remove oversize solids too large to be fed to the pipeline; PA1 pumping the screened slurry to the extraction plant through several kilometers of pipeline; and PA1 feeding the slurry directly into the PSV. This procedure relies on: PA1 the cyclofeeder successfully mixing the oil sand with the water in pre-determined proportions at high rates while simultaneously entraining some air within the slurry, thereby producing an aerated slurry having a pre-determined density; and PA1 the pipeline providing ablation and retention time during which oil sand lumps are disintegrated and bitumen flecks coalesce and coat or attach to the air bubbles, so that the slurry is conditioned and ready to go directly into the PSV and yield the required viable froth yield and quality. PA1 mixing oil sand with unheated water at the mine site using a dredging procedure to produce a low density, ambient temperature slurry; PA1 pumping this slurry through a pipeline to an extraction plant; PA1 adding air (1 to 1.5 volumes of air/volume of slurry) to the slurry in the pipeline; and PA1 adding flotation aid chemicals (specifically a collector having the characteristics of kerosene and a frother having the characteristics of methyl-isobutyl-carbinol ("MIBC") ) to the slurry while in the pipeline to assist in later flotation in a PSV. PA1 dry mining the oil sand; PA1 mixing the as-mined oil sand with water in predetermined proportions near the mine site to produce a slurry containing entrained air and having a controlled density in the range 1.4 to 1.65 g/cc and a temperature in the range 20-35.degree. C.; PA1 pumping the slurry through a pipeline having a plurality of pumps spaced along its length, the pipeline being connected to feed a primary separation vessel ("PSV"); PA1 preferably adding air to the slurry as it moves through the pipeline, more preferably after the last pump, in an amount up to 2.5 volumes of air per volume of slurry, to form an aerated slurry; PA1 introducing the slurry into the PSV, preferably so as to provide an area loading greater than about 4.78 tonnes of oil sand/hour square meter, more preferably in the range of about 4.78 to 9.91 t/h/m.sup.2 and producing bitumen froth, tailings and middlings; and PA1 separately removing the froth, tailings and middlings from the PSV. PA1 the oil sand is dry mined and mixed at the mine site with water using means such as a cyclofeeder to produce a dense slurry having a low temperature; PA1 if the oil sand is of average or higher grade, we have discovered that it can be pipelined in the form of a dense, low temperature slurry, preferably with added aeration but without addition of flotation aid chemicals, and then subjected to flotation in a PSV to give viable primary bitumen recovery in the form of froth having viable quality; and PA1 the dense, low temperature slurry can be fed at loading in the order of about 4.78-9.91 t/h/m.sup.2 into the PSV and still produce the desired froth, thereby maintaining the high density nature of the process. PA1 operating the slurrying and pipelining steps at a density in the order of about 1.6 g/cc and a temperature in the order of 25.degree. C.; maintaining the slurry area loading to the PSV within generally defined limits to ensure a vessel of adequate diameter so as to facilitate bitumen flotation; PA1 pumping the slurry through a pipeline having sufficient length so that the retention time is at least 4 minutes, to achieve conditioning; PA1 adjusting the density of the flotation step by adding flood water to the slurry as it approaches the PSV to reduce its density to less than 1.5 g/cc; PA1 venting excess air from the slurry as it is being introduced into the PSV through a vent stack associated with the incoming feed distributor; and PA1 adding sufficient heated water as an underwash layer between the froth and middlings in the PSV to ensure production of froth having a temperature greater than about 35.degree. C. PA1 operating the slurrying and pipelining steps at low temperature and high density; and then PA1 moderating density at the PSV, if required, to promote effective flotation; maintaining slurry loading within limits to promote effective flotation; PA1 using an underwash of hot water to heat the froth and enable it to flow more easily; and PA1 modifying the PSV step to cope with the large air content in the slurry and minimize turbulence. PA1 a well mixed, high density, low temperature slurry of average quality oil sand, PA1 will condition adequately in a pipeline so as to yield viable primary recovery of bitumen in the form of froth of viable quality, particularly if the steps of air addition, excess air venting, slurry dilution and slurry loading are incorporated, without the addition of flotation aid chemicals, and PA1 the froth can be heated to at least 35.degree. C. by use of a hot water underwash in the PSV, thereby assisting in removing the froth from the PSV and satisfying downstream froth temperature needs. PA1 flotation aid chemicals are added to the slurry in the pipeline; and PA1 secondary recovery of bitumen by way of flotation with agitation and submerged aeration is practiced.
The middlings from the deep cone vessel are further processed in air flotation cells to recover contained bitumen.
It is important to note that the process temperature in the tumbler and PSV is in the order of 80.degree. C. This high slurry temperature is used to reduce the bitumen viscosity sufficiently so that it will readily separate from the sand and coat the air bubbles in the aeration process. It also serves to enhance the density difference between bitumen and water, which leads to more effective flotation separation. The high temperature also promotes faster disintegration of the oil sand lumps in the tumbler and faster coalescence of the bitumen flecks in the PSV.
It is well understood in the industry that the quality of the oil sand has very significant effects on the completeness of primary bitumen recovery in the PSV and the quality of this froth (the froth from the PSV is termed "primary" froth - that from the secondary circuit is termed "secondary" froth). The quality of the useful oil sand produced from a mine will vary in grade. The present invention is directed to establishing processes which are capable of treating "low grade" and "average" oil sands to yield viable bitumen recovery and froth quality at a lower energy input than the current commercial processes. A "low grade" oil sand will contain between about 7 and 10 wt. % bitumen. An average oil sand will contain at least 10 wt. % bitumen, typically around 11 wt. %.
To be useful, a new or modified process for extracting bitumen from low grade and average oil sands should achieve a total recovery value falling within the extraction recovery curve set forth in FIG. 1.
A fairly recent and major innovation in the oil sand industry has involved:
This innovation is disclosed in Canadian Patent No. 2,029,795 (Cymerman et al) and U.S. Pat. No. 5,039,227 (Leung et al), both assigned to the present assignees and incorporated herein by reference.
The cyclofeeder operates on the principle of recycling part of the produced slurry and introducing it tangentially into the vessel to produce a vortex. The oil sand is delivered into the vortex. Water is added to the vortex, to maintain the consistency of the slurry. An alternative to the cyclofeeder is the trough system described in U.S. patent application Ser. No. 08/787,096, also incorporated herein by reference.
The innovation has enabled remote satellite mines to feed a central extraction plant and has substantially eliminated conveyors and tumblers from the process equipment.
Another innovation was developed by the OSLO group of companies. This process involves:
This process is disclosed in a paper "Dredging and cold water extraction process for oil sands" by W. Jazrawi, delivered at a seminar convened in March, 1990, by the Alberta Oil Sands and Technology Authority and U.S. Pat. No. 4,946,597 (K. N. Sury).
The OSLO process differs from the commercial hot water process and the mixing/pipelining process in that it is carried out at ambient temperature. Water at ambient temperature is used for slurry instead of expending energy to heat water and then having to convey the hot water to the mine site in an insulated pipeline.
The Jazrawi paper describes testing slurries having densities of 25 wt. % and 50 wt. % by weight solids in a pipeline test facility. However, the stated slurrying process, dredging, offers little control over slurry density and no control over temperature. Dredged oil sand slurry typically has a density in the order of 1.2 to 1.3 g/cc. At this order of density, the process may lose viability as a large volume of slurry has to be moved through the line and processed to treat a specific quantity of oil sand. In addition the oil sand loading of the PSV surface area will necessarily be low, leading to the need for a very large PSV surface area.
The OSLO process also differs from the hot water process in that it is thought that the bitumen flecks tend to attach to the air bubbles, rather than coating them. The intimation is that, at low temperature, the bitumen is solid-like rather than fluid in nature. The flotation aid chemicals are provided to enhance the attachment mechanism. The Jazrawi paper indicates that the dosage of flotation chemicals should increase as the grade of the oil sand decreases.
With this background in mind, the present invention is now described.